1. Field of the Invention
The present invention relates to a chip bump structure and a method for forming the same.
2. Description of the Related Art
Flip-chip bonding technology is defined as the technique of directly connecting chips to substrates using bumps. Recently, the input/output terminals of such technology are getting smaller, resulting in densely concentrated pads on chips. Due to the dimension limitation of conventional bumps, bumps are not suitable for fine-pitch chips. Consequently, copper pillars have been introduced.
Copper pillars are formed using an electroplating process, obtained by depositing metal in via holes in a thick photoresist layer. However, a uniformly thick photoresist layer cannot be easily obtained, which means a uniform height of copper pillars cannot be easily achieved. Non-uniform heights of copper pillars may cause poor electrical connections between chips and a printed circuit board. Further, copper pillars are hard, and when the copper pillars having different heights are bonded, taller copper pillars may sustain greater stresses, causing a crack generation or peel-off issue. In addition, the formation of via holes on a thick photoresist layer requires high exposure energy. Such a photolithography process cannot be easily controlled. Furthermore, when via holes are formed by development, the formation of via holes may not be completed due to underdeveloped photoresist, adversely affecting the formation of copper pillars. Moreover, when the under bump metal layer is etched, the copper pillars may also be etched, rendering the dimensions of the copper pillars impossible to control.
In addition, copper pillars are easily oxidized, causing problems with electrical connection between chips and printed circuit boards.
In view of the issues related to the adoption of copper pillars as bump structures, a new bump structure is required as a replacement.